1. Technical Field
The present invention relates to a circuit arrangement for an optical receiver used as an optical-to-electrical transducer in optical communication systems.
2. Background of the Invention
If great band-width and high dynamic range are recruited, a transimpedance amplifier is frequently used as the preamplifier, see Dieter Lutzke, "Lichtwellenleiter-Technik", Pflaum Verlag M/e,uml/u/ chen, 1986, pp. 282 to 289. FIG. 1 shows a generally known equivalent circuit of an optical receiver. The photodiode PD is shown as a current source I.sub.Ph shunted by a capacitance C.sub.D, and the succeeding transimpedance amplifier TIV consists of an operational amplifier V with a feedback resistance R.sub.F, an input resistance R.sub.V, and an input capacitance C.sub.V. The photodiode PD delivers a photocurrent I.sub.Ph which, to a first approximation, is proportional to the received light power. The operational amplifier, whose output is fed back through the feedback resistance R.sub.F, converts the photocurrent I.sub.Ph into an output voltage U.sub.a. Assuming a high gain v, the corresponding current-voltage transfer function of the transimpedance amplifier is ##EQU1## follows that the 3-dB cutoff frequency is ##EQU2##
The feedback resistance R.sub.F is thus fixed by specifying a desired bandwidth. However, the resistance value influences the noise characteristic and, hence, the receiver sensitivity. If the receiver sensitivity is to be high, the feedback resistance R.sub.F must be made as large as possible. This reduces the noise contribution of the feedback resistance R.sub.F, but also the bandwidth of the optical receiver.
In prior art solutions, the feedback resistance value is chosen in accordance with the maximum required transmission rate in an optical communication system, see R. G. Meyer, R. A. Blauschild, "A Wide-Band Low-Noise Monolithic Transimpedance Amplifier", IEEE Journal of Solid-State Circuits, Vol. SC-21, No. 4, Aug. 1986. If a receiver is used at a lower transmission rate, the mismatched feedback resistance will cause a loss of receiver sensitivity.